The present invention relates to a dielectric ceramic composition for high frequency with high dielectric constants, high quality factor (Q) values, and stable temperature coefficients in the resonant frequencies.
More specifically, the present invention relates to a dielectric ceramic composition for high frequency that can easily obtain a high quality factor (Q) value by sintering for a shorter time at a lower temperature.
There have been recent remarkable developments in communication systems that use microwave frequencies with a range of 300 MHz to 300 GHz in mobile communications such as wireless telephone sets and car-phones, satellite broadcastings, satellite communications and the like. For the practical use of such new media, dielectric ceramics for high frequency are now increasingly applied to resonators, band pass (or stop) filters, duplexers and microwave integration circuit (MIC) and the like.
High dielectric constant and high frequency are required for miniaturization of parts because the wavelength of microwaves within dielectric ceramics is inversely proportional to the square root of a dielectric constant, and the frequency. Generally, a dielectric constant (∈r) is inversely proportional to a quality factor (Q) value, and as the frequency in use is increased to xe2x89xa72 GHz, the parts are sufficiently miniaturized. Thus, dielectric ceramics materials having high quality factor rather than large dielectric constant, and which have excellent selectivity and band width are required. Dielectric ceramics materials having dielectric constant of 90 are used in cellular phone filters of 800 MHz bandwith and those having dielectric constant of 38 are used in PCS phone filters of 1.9 GHz bandwith. As the high frequency may be used in WLL (Wireless Local Loop), IMT 2000 (International Mobile Telecommunication for the 2000s), UMTS (Universal Mobile Telecommunication System) and the like in the future, the materials having high quality factor and dielectric constant of 20 will be utilized and are now actively applied to a variety of resonators, filters for base stations, antennae and the like.
The representative examples of dielectric ceramics materials having high Q values, which have been developed, include MgTiO3xe2x80x94CaTiO3 type, of which the Qxc2x7f0 value is 56,000 GHz, the dielectric constant is 21, the temperature coefficient of resonant frequency is 0 ppm/xc2x0 C., and the sintering temperature is about 1,400xc2x0 C. [See, K. Wakino, Ferroelectrics, 91, 69 (1989)]. The ceramics materials having the Qxc2x7f0 value of 56,000 GHz can be applied to some parts for the mobile communications but have some problems in the frequency selectivity to be used as the materials for 3rd generation mobile communications of 3-4 GHz. Therefore, a material having higher Q value is needed.
Another dielectric ceramics materials having high Q value are Ba(Zn,Ta)O3 type, of which the Qxc2x7f0 value is 150,000 GHz, the,dielectric constant is 30, the temperature coefficient of resonant frequency is 1 ppm/xc2x0 C., and the sintering temperature is about 1,600xc2x0 C. [See, S. B. Desu. J. Am. Ceram. Soc., 68, 546 (1985)]. The composition has a high Q value but has problems of very high sintering temperature and to obtain the high Q value, and it is necessary to achieve an ordering of the B site by heat treating for the long time (xe2x89xa710 hours) in high temperature.
The object of the present invention is to provide solutions to the aforementioned problems in the art and provide a dielectric ceramic composition for microwave frequency that have a high quality factor (Q) value without any necessity for achieving an ordering of the B site by heat treating for lengthy times in high temperature, and has a high Qxc2x7f0 value, a high dielectric constant and a stable temperature coefficient of resonant frequency.